The invention relates to the field of wireless telecommunications networks, and in particular to wireless local networks complying with the IEEE 802.11 family of standards.
Such networks are also known as WiFi networks. In numerous applications, they are used to network together various stations, e.g. computers, personal assistants, and peripherals.
In the document “IEEE 802.11a-1999, IEEE 802.11b-1999, IEEE 802.11d-2001, part 11: wireless medium access control (MAC) and physical layer (PHY) specifications”, the 802.11 standard defines a method of regulating traffic in the wireless network. That method makes use of a system of congestion windows (CW) for regulating the traffic. According to that standard, and in general manner, a station draws a random number cw in the range 0 to CW−1, the value CW being an integer lying between two values CWmin and CWmax defined by the 802.11 standard, e.g. 16 and 1024. This value cw is used as a backoff timer for transmitting a packet, with counting being deferred if the station observes that another station is transmitting.
The main steps of that conventional method are described below with reference to FIGS. 1 and 2.
When a frame TR is ready for transmission (step A5), the station determines (step A10) whether the medium has been free for a duration of at least DIPS.
If so, the frame is transmitted (step A15).
Otherwise, and in particular if the station itself was transmitting a frame, the station starts (step A20) a contention period after a silence of duration DIPS since the last transmission.
The contention period consists in waiting (step A30) for the above-mentioned backoff timer cw drawn at random in the range 0 to CW−1 (step S25) before transmitting the frame TR (step A35). On each attempt at transmission that is unsuccessful, i.e. when the transmitted frame is not acknowledge (test A40), a new congestion window CW is calculated on the basis of the current value of CW (step A35) and using the formula CW′=Min(CWmax, 2.CW), and a new backoff timer is drawn at. random in the range 0 to the new value CW′−1 (step A40) .
As shown in FIG. 2, contention is suspended for a total period including both a period of the medium being active with a frame being transmitted, and also a period DIFS (DCF interframe space) that is long enough to leave the receiving station time to acknowledge the received frame after SIFS (short interframe space) and to allow an access point to transmit a management message after PIFS (PCF interframe space). The stations performing that mechanism do not transmit during the range [PIFS, DIFS] that is referenced P in this figure.
Unfortunately, that conventional mechanism with congestion windows generates a large number of collisions, which means that the user suffers a considerable loss of bandwidth.
Several contention resolution methods have been proposed in the state of the art.
One method is known as the “tournament” method and is described in the document by Z. Abichar and M. Chang “CONTI: Constant time contention resolution for WLAN access” IFI Networking 2005, referred to below as [CONTI]; it proposes a constant time method of resolving contention that makes use of a series of successive tests for selecting the station that is to transmit, for regulating the transmissions of frames, and for limiting the collision rate.
In general manner, the tournament method consists in organizing a kind of tournament between stations having at least one frame for transmission. A tournament is made up of a certain number of selection rounds, each executed during a time interval of predefined duration. At the beginning of the tournament, all of the stations having a frame for transmission could be entitled to transmit the frame. On each selection round, one or more stations might be deleted from the list of stations entitled to transmit depending on whether they are allocated an entitlement to transmit or a prohibition to transmit during the selection rounds. At the end of the tournament, only those stations that have not been eliminated are entitled to transmit. If a plurality of stations are still in the running at the end of the tournament, they transmit simultaneously, thereby giving rise to a collision, and thus to disturbed reception with it being impossible to receive the transmitted data frame correctly. These stations must then participate in the following tournament in a new attempt to transmit the frame.
More precisely, in the CONTI method, the elimination of stations seeking to transmit is accomplished by using a Boolean variable referred to as a “try-bit”. More precisely, each station selects this variable randomly and transmits a signal over the network if this value is equal to 1, and otherwise listens to the network. A station withdraws from the network, i.e. decides not to transmit its data packet during a series of selection rounds, if the binary value is 0 and if it detects a signal being transmitted by another station.
Although providing better performance than the congestion method defined in the 802.11 standard, the contention resolution method proposed by CONTI still generates a large number of collisions in accessing the wireless network. This drawback is due to the fact that the probability relationship used for drawing the random “try-bit” variable is not optimized.
Document FR 2 893 206 in the name of the Applicant describes another tournament method that makes it possible to reduce considerably the number of collisions experienced by CONTI, by improving the probability relationship used for drawing the try-bit binary random value.
In general manner, the problem of coexistence between stations complying with the 802.11 protocol, referred to below as “conventional” stations, and stations using a tournament mechanism raises a problem in terms of fair access to resources.
The invention provides a method of regulating transmission that makes it possible to allow stations that perform a tournament mechanism to cohabit with conventional stations in compliance with the 802.11 protocol.